First, I just want to say thank you for the themes that you have come up with so far. I know that you are not going to make everyone happy, and I appreciate the fact you still go through with certain themes while knowing this.
My suggestion would be to have new season themes be paired with a season theme from the past or a new one, in a manner where we have one gameplay theme and one mechanics/gearing theme.
For instance, we could have the elemental theme this season, while also have a convention of elements buff permanently active for the season. Or in a future season you could create a new gameplay theme, but also provide all players with the RoRG buff again.
I just think it would be cool to see themes evolve a little, and I think the best way to do it given resources is to make sure that every season has not only a gameplay theme like triune or season of elements, but it also has a gearing/mechanic theme like RoRG or LoD.
*Edit - I think it would be extremely beneficial to add my suggested Convention of Elements buff to the current seasonal theme.